1. Field of the Invention
The invention relates to analog by digital multipliers capable of operating in all four quadrants of analog and digital sign combinations.
2. Description of the Prior Art
In situations which require the multiplication of an analog value by a digital value, as, for example when real-time analog data is to be modified according to stored digital data, there is a requirement for an analog by digital multiplier. Analog by digital multipliers of the prior art have routinely performed such multiplications for a particular combination of digital and analog signs. For example, the multiplication has been performed provided that the digital and analog values are both positive. However, there are four possible combinations of analog and digital signs: both may be positive; both may be negative; the digital may be positive while the analog is negative; or the analog may be positive while the digital is negative. These four possibilities are commonly referred to as the four quadrants of analog by digital multiplication.
Analog by digital multipliers which are operable in all four quadrants have been developed in the prior art. Some prior art multiplier devices have utilized an operational amplifier for the purpose of multiplying the two values. These devices generally achieve multiplication by the control of a resistance located at one of the inputs to the operational amplifier. Another approach has been to carry a particular sign bit along with the digital information and to supply the binary data in its complement form for multiplication. Some four quadrant multipliers involve the use of two quadrant multipliers which are made to behave as four quadrant devices as by adding a constant voltage to a variable that is not permitted to change signs. In other prior art four quadrant multipliers, the sign bit of the digital value is used for switching purposes at the output of an operational amplifier.